System and method of rank adaptation in mimo communication system

ABSTRACT

An apparatus may include a transmitter arranged to wirelessly transmit channel status reports for channels within a transmission band to a base station and a processor. The apparatus may further include a rank adaptation (RA) module operable on the processor to direct the transmitter to send a multiplicity of sub-band channel quality indicator (CQI) reports, each sub-band CQI report comprising a measurement of a respective sub-band of the transmission band and a multiplicity of rank indicator (RI) reports, where each sub-band CQI report is accompanied by an RI report. The apparatus may further include a digital display arranged to display information transmitted via the base station to the apparatus. Other embodiments are disclosed and claimed.

This application claims priority to U.S. provisional patent applicationSer. No. 61/481,024, filed Apr. 29, 2011, and incorporated by referenceherein in its entirety.

BACKGROUND

The use of multiple input multiple output (MIMO) technology hasattracted increased attention for use in wireless communications systemsbecause MIMO offers significant increases in data throughput and linkrange without requiring additional bandwidth or transmit power. Theincreased performance afforded by MIMO technology stems from higherspectral efficiency (greater number of bits transmitted per second perHertz of bandwidth), as well as greater link reliability or diversity.Accordingly, MIMO forms an important part of modern wirelesscommunications standards including 3GPP Long Term Evolution (see 3GPP TS36.213, section Technical Specification Release 10, June 2011, 3rdgenerationPartnership Project), IEEE 802.11n (WiFi), 802.16 (WiMAX) andHSPA+.

One area of concern is the ability to provide robust rank adaptation.Rank adaption refers to the dynamic control of rank according tochanging channel conditions. The channel conditions may be determined bysuch parameters as signal to interference and noise ratio (SINR) andfading correlation between antennae in a MIMO system. With the use ofspatial multiplexing, a base station (or eNodeB, or eNB) may sendmultiple data streams or layers to UEs in a downlink transmission usingthe same frequency. The number of such layers or streams is defined asthe rank. The UE may periodically measure a channel and send arecommendation of the rank to the eNB. The so called rank indicator (RI)may be sent periodically or aperiodically in different schemes. Becausethe RI reported to the eNB may change with time, the eNB may adjust thenumber of data streams used in a downlink transmission, based upon thechanging RI received from the UE. However, several factors may renderthis process less than ideal. In some circumstances, the interferencelevels that may affect channel quality can change substantially betweentwo successive RI reports, in which case, the eNB has no occasion toadjust the rank even though the last reported rank may not beappropriate due to the changed interference conditions. In othercircumstances, when a so-called wideband rank is used, the rankindicator reported may be based upon an entire transmission band(wideband), which may be composed of a group of frequency sub-bands usedfor communications between the UE and eNB. In many cases, theinterference conditions may vary substantially between differentsub-bands within the wideband, thereby compromising the validity of awideband RI reported by the eNB for individual sub-bands.

Another concern has been raised regarding the use of multiuser MIMO(MU-MIMO) where a UE may transmit precoding matrix indicator/channelquality indicator (PMI/CQI) reports with too high a rank to effectivelysupport the most efficient MU-MIMO scheduling: In MU-MIMO an eNB mayschedule multiple different UEs for transmission over the sametransmission band. The reporting of an excessively high rank may ariseas a consequence of the fact that the UE can only evaluate its own linkperformance, and in general is not aware of any co-scheduling candidatesin the MU-MIMO scheme. As a consequence, even though the linkperformance to a UE may be maximized by a high rank single user MIMO(SU-MIMO transmission), the system performance may very well be higherif the UE and a second terminal (unbeknownst to the UE) are co-scheduledusing lower rank transmissions. The reported information includingPMI/CQI/RI may therefore be ill-matched to a MU-MIMO allocation. Inparticular, the terminal may often select a too high transmission rankto be beneficial for MU-MIMO scheduling.

In other circumstances, where the eNB prefers operating under SU-MIMOtransmissions, the UE may report a rank of 1, although the eNB mayprefer the UE to report an adapted rank, rather than always reporting arank of 1.

It is with respect to these and other considerations that the presentimprovements have been needed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates one embodiment of a communications system.

FIGS. 2 a and 2 b depict operation of a UE according to variousembodiments.

FIGS. 3 a and 3 b compare conventional architecture and an enhancedarchitecture for providing channel status reports in accordance withsome embodiments.

FIG. 4 depicts one embodiment of a CSI report structure that may be usedto aperiodically report PMI/CQI/RI information.

FIG. 5 depicts an embodiment of an aperiodic CSI report structure havingsub-band RI reports.

FIG. 6 depicts an embodiment of a UE.

FIGS. 7 a and 7 b present a comparison of known rank 1 and rank 2 PMIdistributions.

FIG. 8 depicts a logic flow in accordance with present embodiments.

FIG. 9 depicts another logic flow consistent with the presentembodiments.

FIG. 10 a depicts another logic flow consistent with furtherembodiments.

FIG. 10 b depicts a further logic flow consistent with otherembodiments.

FIG. 10 c depicts a further logic flow consistent with otherembodiments.

FIG. 11 illustrates an embodiment of an exemplary computingarchitecture.

FIG. 12 illustrates a block diagram of an exemplary communicationsarchitecture.

FIG. 13 is a diagram of an exemplary system embodiment.

DETAILED DESCRIPTION

Various embodiments may be generally directed to systems that employwireless communications using multiple input multiple output (MIMO)wireless communications. Some embodiments may be particularly directedto apparatus, architecture and methods for rank adaptation.

Various embodiments may comprise one or more elements. An element maycomprise any structure arranged to perform certain operations. Althoughan embodiment may be described with a limited number of elements in acertain arrangement by way of example, the embodiment may include moreor less elements in alternate arrangement as desired for a givenimplementation. It is worthy to note that any reference to “oneembodiment” or “an embodiment” means that a particular feature,structure, or characteristic described in connection with the embodimentis included in at least one embodiment. The appearances of the phrase“in one embodiment” in various places in the specification are notnecessarily all referring to the same embodiment.

In one embodiment, a method comprises measuring, over a wirelesstransmission band, a multiplicity of sub-band channel quality indicators(CQI), each sub-band CQI corresponding to a respective frequencysub-band of the transmission band; transmitting one or more sub-band CQIof the multiplicity of sub-band CQIs at a first instance; selecting oneor more rank indicators (RI) for the transmission band; andtransmitting, at the first instance, one or more RI reportscorresponding to the selected one or more RIs.

The method may also include transmitting each sub-band CQI reportperiodically.

Alternatively, the method may further include transmitting amultiplicity of sub-band PMI reports, each sub-band PMI reportcorresponding to a respective sub-band CQI report.

The method may also include transmitting a sub-band RI report with arespective sub-band CQI report, each sub-band RI report and respectivesub-band CQI report corresponding to a same sub-band of the transmissionband.

The method may additionally include transmitting the sub-band CQIreports aperiodically, which in a first implementation, may involveselecting from within the transmission band a multiplicity of band partrank indicators, each band part RI based upon a measurement of amultiplicity of contiguous sub-bands and transmitting, with the sub-bandCQI reports, a multiplicity of band part RI reports each derived from aselected band part RI; while in a second implementation, may involveselecting from within the transmission band a multiplicity of sub-bandrank indicators each based upon a measurement of a respective frequencysub-band of the transmission band, and transmitting, with the sub-bandCQI reports, a multiplicity of sub-band RI reports each containing arespective sub-band RI.

FIG. 1 illustrates a block diagram of one embodiment of a communicationssystem 10 that may include embodiments of the channel estimationarchitecture disclosed herein. As shown in FIG. 1, the communicationssystem 10 may comprise a network 12 that communicates over links 18-mwith a plurality of nodes 14-n, where m and n may represent any positiveinteger value. In various embodiments, the nodes 14-n may be implementedas various types of wireless devices. Examples of wireless devices mayinclude, without limitation, a station, a subscriber station, a basestation, a wireless access point (AP), a wireless client device, awireless station (STA), a laptop computer, ultra-laptop computer,portable computer, personal computer (PC), notebook PC, handheldcomputer, personal digital assistant (PDA), cellular telephone,combination cellular telephone/PDA, smartphone, pager, messaging device,media player, digital music player, set-top box (STB), appliance,workstation, user terminal, mobile unit, consumer electronics,television, digital television, high-definition television, televisionreceiver, high-definition television receiver, and so forth.

In some embodiments, a multiplicity of devices in communications system700 may employ multiple input and multiple output (MIMO) communicationsin which both receiver and transmitter employ multiple antennae. Someembodiments of a communications system may be implemented with a radiotechnology such as IEEE 802.16 (WiMAX), IEEE 802-20, evolved UTRA(E-UTRA), etc. IEEE 802.16m is an evolution of IEEE 802.16e, andprovides backward compatibility with an IEEE 802.16-based system. TheUTRA is a part of a universal mobile telecommunication system (UMTS).3rd generation partnership project (3GPP) long term evolution (LTE) is apart of an evolved UMTS (E-UMTS) using the E-UTRA. LTE-advance (LTE-A)is an evolution of the 3GPP LTE.

For clarity, the following description may focus on embodiments relatedto LTE-A. However, other embodiments may employ other standards, asnoted above.

FIGS. 2 a and 2 b depict operation of a UE 100 according to variousembodiments. The UE 100 may be scheduled to operate in either an SU-MIMOmode (FIG. 2 a) or an MU-MIMO mode (FIG. 2 b). In one implementation ofan SU-MIMO mode depicted in FIG. 2 a, an eNB 102 may transmit downlinksignals 106 from each of two antennae 114, 116. As illustrated in FIG. 2a, each antenna 114, 116 transmits a signal to each of two UE antennae110, 112. However, in some embodiments, UE 100 and eNB 102 may each haveadditional antennae.

UE 100 may also operate in MU-MIMO mode, as depicted in FIG. 2 b. UE 100and a separate terminal (UE 104), which may operate in a conventionalmanner, are each depicted as transmitting respective signals 124 and 126that form a portion of uplink signals 122 received by eNB 102 during anuplink communication. In the uplink communication depicted, signals 124are sent from antenna 112 and may be received at antennae 114 and 116 ofeNB 102. The signals 124 may be provided over a physical uplink controlchannel (PUCCH) in various embodiments. Similarly, the additional UEdevice 104 transmits signals 126 from antenna 118, which may also bereceived at antennae 114, 116 of eNB 102. However, other configurationsof MU-MIMO are possible.

In various embodiments, the uplink signals 124 sent from UE 100 mayinclude control signals such as RI, PMI, and CQI, among others. At leastsome of the control signals transmitted by UE 100 may be reported in aregular periodic fashion or in an aperiodic fashion in differentembodiments. In various embodiments, UE is arranged to modifycommunications with eNB in order to provide for robust rank adaptation.For example, UE 100 may operate in an environment in which interferencechanges substantially over time. The interference may take place in arapid an unpredictable manner. It may therefore be desirable to changethe rank reported to the eNB in a timely fashion to account forsubstantial interference changes that may alter the preferred rank. Inaccordance with various standards, the framework for reporting ofcontrol signals, which may include channel status information (CSI) suchas PMI/CQI/RI reports (also termed “channel status reports” hereinafter)may limit the flexibility in reporting RI information. In someembodiments, the UE may provide a more effective framework for channelstatus reports that updates RI in a more effective manner. The UE 100may include, for example, a rank adaptation (RA) module 108, which mayperform various functions such as determining rank information to bereported and scheduling rank indicator reports, as detailed below. Inparticular RA module 105 may implement the procedures and architecturedepicted in the FIGS. 3-10 and discussed below.

FIGS. 3 a and 3 b depict operation of an enhanced framework for channelstatus reports in accordance with some embodiments. In. FIG. 3 a, aframework for providing channel status reports is illustrated based uponthe timing specified in LTE release 10. In particular, the scenarioshown depicts CQI/PMI/RI timing as specified for PUCCH 2-1 sub-bandreporting mode. The uplink communication is divided into nine contiguoussub-bands 154 that comprise a transmission band. The sub-bands 154 maybe grouped into three band parts 156. At time t₀, a wideband (W2) rankindicator report 150 and CQI/PMI report 152 (“PMI” is not explicitlyshown in the label for clarity) are transmitted consistent with widebandchannel conditions representing the frequency range FR of thetransmission band. When the RI report 150 is transmitted to an eNB, thedownlink rank may be updated in accordance with the reported RI. Forexample, the rank may be rescheduled from “2” to “4” after RI report 150is received.

In accordance with the PUUCH 2-1 sub-band reporting mode, a series ofsub-band CQI/PMI reports 158 a-158 f are transmitted over a subsequenttime period corresponding to reporting period for the sub-band CQI/PMImeasurements. Each CQI/PMI report 158 a-f corresponds to a differentsub-band of sub-bands SB0-SB8 and is reported at a different time thateach other sub-band. As depicted in FIG. 3 a, at time t₁, aninterference event I takes place. For example, a strong new interferermay appear within the frequency range of W2. However, to conform to thereporting procedures specified by the LTE 2-1 sub-band reporting modestandard, a subsequent RI report may not be transmitted until at least atime t₂, which corresponds to the end of the current reporting periodfor all the CQI/PMI sub-band reports to be transmitted. Thus, during theentire time period 160 between t₁ and t₂, the rank may remain at “4,”although interference conditions may warrant a change, such as a changeto a rank of “1.” Accordingly, communications may be degraded duringthis period 160.

In an embodiment depicted in FIG. 3 b, the channel status reportframework (architecture) is modified, for example, using an RA module105, which may provide more timely RI adaptation. In this embodiment, aseries of sub-band reports 168 a-168 f are arranged with a similartiming to CQI/PMI sub-band reports 158 a-f. However, an RI report 170a-170-f is included in each respective sub-band report 168 a-f. Thus, attime t₃, RI report 170 b is provided, which may update the rank to “4”in one example. In the scenario in which a change in interferenceconditions takes place at time t₁ the channel status report architectureof the embodiment of FIG. 3 b provides the ability to adapt the rank ina more timely fashion. As illustrated, an immediately subsequent RIreport 170 c may be provided at time t₄. Based upon channel measurementsat time t₄, which reflect the change in interference that occurred attime t₃ and may persist, the UE may recommend a rank of “1” in the RIreport 170 c. Thus, the duration 180 between time t₁ and t_(4, in) whichthe operating rank (4) may be ill-suited to the present channelconditions, is reduced to a much smaller duration 180 than the duration160 using the conventional framework of FIG. 3 a.

In some embodiments, using the architecture generally depicted in FIG. 3b, the RI report that precedes the sub-band CQI/PMI reports may bedesigned as a wideband rank indicator, while in other embodiments, theRI report may be a sub-band RI that is applicable only to the given bandpart that is scanned by the sub-band CQI-PMI report.

In other embodiments in which CSI is provided aperiodically, thereporting of RI may be tailored to the band structure of the CSIreports. For example, CSI may be provided over a physical uplink sharedchannel (PUSCH) in an aperiodic fashion. FIG. 4 depicts one embodimentof a CSI report structure 204 that may be used to aperiodically reportPMI/CQI/RI information. The report structure may be considered to amodification to PUSCH 3-2 report mode. As illustrated, the reportstructure 204 comprises a series of CSI sub-bands corresponding to thefrequency sub-bands SB₀-SB₉ of frequency band 200. As illustrated, acorresponding PMI set of sub-band reports 206, comprising SB PMI₀-SBPMI₈ and CQI sub-band reports 210 comprising SB CQI₀-SB CQI₈ is providedfor each sub-band SB₀-SB₉. In addition, a set of three RI reports (bandpart RI) 208 a-208 c are provided that each span three contiguoussub-bands. These RI reports accordingly provide RI spanning thefrequency range defined by the three contiguous sub-bands in each case.

Alternatively, aperiodic RI reports may be provided for each sub-band ofa frequency band. FIG. 5 depicts an embodiment of a CSI report structure212 that includes nine separate sub-band RI reports 214 a-214 i eachcorresponding to a sub-band frequency SB₀-SB₈. The band-part or sub-bandrank indicators shown in the embodiments of respective FIGS. 4 and 5 mayfacilitate more accurate rank adaptation by providing aperiodic RIreports that cover a more narrow frequency range within a wideband andare therefore more likely to report a rank that accurately reflects thechannel conditions within a given frequency range, which may differsubstantially from those conditions in other frequency ranges within thesame transmission band.

In further embodiments, a UE may be arranged to modify the searchprocess for selecting a PMI to be reported to the eNB and the processfor determining a best rank indicator to report. FIG. 6 depicts anembodiment of a UE 100 that includes processor 220, memory 222, andtransmitter 224. The antennae 110, 112 may serve both as transmitter(Tx) and receiver (Rx) antennae. Memory 222 may include a codebook(s)226, which may include multiple ranks, for example 8 ranks. In someembodiments, the codebook 226 may be arranged generally as provided forin LTE release 10. In particular, codebook 226 may have a nestedstructure, such that for each precoder matrix of a given rank thereexists at least one corresponding column in all codebooks of ranks lowerthan the given rank.

In one embodiment, the UE 100 may perform rank adaptation according tothe following procedure. The UE 100 may perform channel measurements todetermine various parameters described below. The processor 220 mayperform a PMI search using codebook 226 according to

i _(r)=^(argmax) _(v) _(i) _(εC) _(r) (trace(v _(i) ^(H)(Rv _(i)))  (1)

where C_(r) denotes a codebook having rank r, R is the measured channelcovariance matrix for a given band, and i_(r) denotes the best PMI forrank r. After the best PMI for a given rank r is selected in accordancewith equation (1), the UE may select the best rank r_(best) to report toa base station.

In one embodiment, r_(best) is determined according to

$\begin{matrix}{r_{best} = {\,_{0 < r < r_{\max}}^{argmax}\left( {{capacity}\left( {v_{i_{r}},H,{SINR}} \right)} \right)}} & (2)\end{matrix}$

where H is the channel matrix of interest and SINR is thesignal-to-noise-and-interference ratio per each Rx antenna. Thus, afterdetermining the SINR, the UE can calculate r_(best) and report both thebest rank and the best PMI for the best rank to an eNB. In this manner,in single user MIMO operation, the capacity of SU-MIMO may be maximizedwhen rank adaptation is performed.

In further embodiments, the UE may perform rank adaptation and PMIselection to enhance MIMO operation in an environment in which dynamicswitching between single user MIMO and multiuser MIMO operation may takeplace. This may improve upon current procedures where the codebook isdesigned for rank 1 PMI searches.

As is known, codebook-based precoding generally involves storing acodebook (i.e. the set of precoding matrices) at both the transmitterand the receiver in advance of a communications session. The receiverthen may follow specified rules to select the optimal precoding matrixaccording to the current channel state and return the PMI of theselected matrix to the transmitter. However, previous codebooks, such asLTE release 8 codebook, may not perform optimally in a MU-MIMO scenarioor for dynamic switching between SU-MIMO and MU-MIMO.

In particular, under current codebook procedures, the best rank 2 PMIneed not equal the best rank 1 PMI. Accordingly, current procedures forrank adaptation where high rank is reported may not result in theoptimum PMI. As an example, when a UE performs rank adaptation andselects rank 2 for reporting, the UE needs to report one precoder andtwo channel quality indicators (CQI), each related to one column of therank 2 precoder. In codebooks having a nested structure, such as the 4Tx codebook specified by LTE release 8, all rank 2 precoders that havethe same PMI value as that of a rank 1 will contain the correspondingrank 1 precoder as the first column in the rank 2 precoder. Although therank 1 precoder coincides with a portion of the rank 2 precoder, thisdoes not guarantee that the best rank 2 precoding matrix index willalways equal the best rank 1 precoding matrix index.

To illustrate this problem further FIGS. 7 a and 7 b present acomparison of PMI distributions as specified by the LTE release 8codebook. In particular, FIG. 7 a shows the rank 1 PMI distribution 240,where the y-axis indicates the relative frequency for a given point ofthe distribution. The x-axis represents the different precoding matrixindices for the codebook. As depicted in FIG. 7 a, the distribution 240is relatively uniform. Between PMI values of zero to seven, theprobability is about 10 to 15% for each index except for “2.” As aresult, when a UE performs a PMI search in rank 1 codebook, there is agreat probability that the principle eigenvector will be chosen.

In contrast, FIG. 7 b presents a rank 2 PMI distribution 250 based uponthe LTE release 8 codebook. In this case, the PMI distribution 250 ismuch more non-uniform than the rank 1 PMI distribution 240. Inparticular, only PMIs “1,” “3,” and “8” have a significant probability,while all other PMIs have a probability of 5% or less. Accordingly, whena UE performs a PMI search in rank 2, there is a reasonable probabilitythat two non-principal eigenvectors may have higher capacity than theprincipal eigenvector taken together with another vector orthogonal tothe principal eigenvector. Thus, when an eNB receives a rank 2 PMIreport and proceeds to extract the principal eigenvector from the rank 2precoder, the eNB may not be able to find the principal eigenvector inthe case of the higher capacity non-principal eigenvectors. Accordingly,rank adaptation between a rank 1 and rank 2 RI may not properly takeplace.

In accordance with further embodiments, the UE may be arranged to ensurethat the rank 2 precoder always contains the principal eigenvector. Inone implementation, this may be accomplished when the UE performs rankadaptation by determining the PMI only assuming rank 1. This is areasonable approach since, as discussed above, all rank 2 precoders thathave the same PMI value as that of a rank 1 by nature contain thecorresponding rank 1 precoder. The RI may then be determined based uponuse of the same PMI.

In order to determine the PMI regardless of the reporting rank, thefollowing procedure may be followed. The UE may calculate the best PMIfor rank 1 i₀ according to

$\begin{matrix}{i_{0} = {\,_{v_{i} \in C_{0}}^{argmax}\left( {{trace}\left( {v_{i}^{H}{Rv}_{i}} \right)} \right)}} & (3)\end{matrix}$

where C₀ is the codebook having rank 1.

In various embodiments, the best PMI for a rank higher than rank 1 maybe assumed to be the same as a rank 1 PMI to take advantage of thenesting structure of codebooks, such as the LTE release 8 codebook,according to

i _(r) =i ₀ ,r≠0  (4).

The UE may then perform rank adaptation according to the procedure setforth in equation (2) above. Thus, the UE assures that the best higherrank precoder always contains the principal eigenvector which can beused for MU-MIMO transmission.

In various embodiments, the above procedure as set forth in Eqs. (3) and(4) may be implemented for different transmission modes between a UE andbase station. For example, the procedure may be applied to all or asubset of those transmission modes that support reporting CQI/PMI/RI(CSI). In one implementation, the LTE RI reporting procedure may bemodified by implementing a change in the standard for reporting CSI. Therecent LTE standard (3GPP TS 36.213 V10.2.0, 6-2011) specifies ninetransmission modes that a UE may employ to report CSI on an uplinkcontrol channel (PUUCH), including transmission modes 4, 8, and 9. Thetext for the section defining periodic CSI reporting (section 7.2.2)currently reads: For transmission mode 4, 8 and 9, the PMI and CQI arecalculated conditioned on the last reported periodic RI. For othertransmission modes they are calculated conditioned on transmission rank1.

In one embodiment, this procedure may be modified as specified in therevised text: For transmission mode 4, 8 and 9, the PMI is calculatedconditioned on rank 1 and CQI are calculated conditioned on the lastreported periodic RI. For other transmission modes they are calculatedconditioned on transmission rank 1. The reported PMI and COI areconditioned on the last reported RI.

In accordance with the disclosed embodiments, PMI search and rankadaptation procedures may be modified and optimized for either SU-MIMOor MU-MIMO operation. In alternate embodiments, an eNB may employ eitherbroadcasting or unicasting to inform the UE about its preference foreither SU-MIMO or MU-MIMO transmission. The eNB may dynamically changeits preference due to changes in traffic conditions. In response, the UEmay dynamically alter the rank adaptation methods employed between onemethod in favour of SU-MIMO and in favor of MU-MIMO. In particular, theUE may choose an optimum PMI search or rank adaptation procedure toemploy, including those procedures outlined above with respect to Eqs.(1)-(4). The UE may base the choice on that procedure deemed to bestmatch the transmission preference indicated by the eNB. In someembodiments, to minimize overhead, the signalling may comprises aslittle as one bit.

Included herein is a set of flow charts representative of exemplarymethodologies for performing novel aspects of the disclosedcommunications architecture. While, for purposes of simplicity ofexplanation, the one or more methodologies shown herein, for example, inthe form of a flow chart or flow diagram, are shown and described as aseries of acts, it is to be understood and appreciated that themethodologies are not limited by the order of acts, as some acts may, inaccordance therewith, occur in a different order and/or concurrentlywith other acts from that shown and described herein. For example, thoseskilled in the art will understand and appreciate that a methodologycould alternatively be represented as a series of interrelated states orevents, such as in a state diagram. Moreover, not all acts illustratedin a methodology may be required for a novel implementation.

FIG. 8 depicts a logic flow 800 in accordance with the presentembodiments. At block 802 a first channel quality indicator istransmitted at a first instance over a first frequency sub-band of atransmission band. In various embodiments, a precoding matrix index mayalso be transmitted during block 802 and subsequent blocks. At block804, a first rank indicator report is transmitted at the first instanceover the first frequency sub-band. At block 806, a second channelquality indicator is transmitted over a second, different frequency-subband at a second instance. The second frequency sub-band may becontiguous with the first frequency sub-band or may be non-contiguous.The first and second frequency sub-bands may belong to the sametransmission band used in an uplink between a UE and base station. Atblock 808, a second rank indicator report is transmitted in the secondfrequency sub-band at the second instance. In various embodiments,additional CQI and RI reports may be transmitted together in othersub-bands at other instances. In this manner, each instance of reportingCQI information over a given transmission sub-band entails reporting ofa rank indicator, which may be adapted between each reporting instance.The rank indicator may correspond to a wideband rank indicator or may bea sub-band RI that is applicable only to the given band part that isscanned by the sub-band CQI/PMI report.

FIG. 9 depicts another logic flow 900 consistent with the presentembodiments. At block 902, a first set of sub-band PMIs is transmittedspanning a first frequency range at a first instance. At block 904 afirst band part rank indicator is transmitted spanning the firstfrequency range at the first instance. At block 906, a second sub-bandPMI is transmitted over a second frequency range at the first instance.At block 908, a second band part rank indicator spanning the secondfrequency range is transmitted at the second instance. In variousembodiments the procedures in blocks 902-908 may be repeated atadditional second, third, fourth instances, and so forth. In someembodiments, the first and second band part rank indicators may eachspan frequency ranges that are greater than the corresponding first andsecond frequency ranges, so that the frequency range spanned by eachband part rank indicator may cover multiple frequency sub-bandscorresponding to multiple PMIs.

FIG. 10 a depicts another logic flow 1000 consistent with furtherembodiments. At block 1002, a best PMI i_(r) is determined according toEq (1) set forth above. The value i_(r) may be determined for multipleranks r. At block 1004, a best rank r_(best) for reporting to basestation is determined according to Eq. (2) set forth above. At block1006, based upon the determination of i_(r) and r_(best) the best rankand the best PMI for the best rank are reported to a base station.

FIG. 10 b depicts a further logic flow 1020 consistent with otherembodiments. At block 1022, the best rank 1 PMI r₀ is determined inaccordance with Eq. (3) set forth above. At block 1024, the best PMIi_(r) is set to be the same as the best rank 1 PMI r₀. At block 1026,rank adaptation is performed to determine the best rank r_(best) forreporting to base station is determined according to Eq. (2). Byadopting the logic flow 1020, when operating in an SU-MIMO mode, a UEmay experience a slight decrease in performance. However, the logic flow1020 may still provide a net improvement in MU-MIMO performance incomparison to the decrease in SU-MIMO that is substantial.

FIG. 10 c depicts a further logic flow 1040 consistent with otherembodiments. At block 1042 a message from a base station is receivedindicating MIMO preference. At block 1044, a combination of rankadaptation/PMI selection procedure is selected to match the MIMOpreference. At block 1046, the chosen rank adaptation/PMI selectionprocedure is implemented and appropriate PMI/RI reported to basestation. In various embodiments, the implementation of rank adaptationand PMI selection follows any appropriate combination of the embodimentsdiscussed with respect to Eqs. (1)-(4) and depicted in FIGS. 7 a-10 b.

FIG. 11 illustrates an embodiment of an exemplary computing architecture1100 suitable for implementing various embodiments as previouslydescribed. As used in this application, the terms “system” and “device”and “component” are intended to refer to a computer-related entity,either hardware, a combination of hardware and software, software, orsoftware in execution, examples of which are provided by the exemplarycomputing architecture 1100. For example, a component can be, but is notlimited to being, a process running on a processor, a processor, a harddisk drive, multiple storage drives (of optical and/or magnetic storagemedium), an object, an executable, a thread of execution, a program,and/or a computer. By way of illustration, both an application runningon a server and the server can be a component. One or more componentscan reside within a process and/or thread of execution, and a componentcan be localized on one computer and/or distributed between two or morecomputers. Further, components may be communicatively coupled to eachother by various types of communications media to coordinate operations.The coordination may involve the uni-directional or bi-directionalexchange of information. For instance, the components may communicateinformation in the form of signals communicated over the communicationsmedia. The information can be implemented as signals allocated tovarious signal lines. In such allocations, each message is a signal.Further embodiments, however, may alternatively employ data messages.Such data messages may be sent across various connections. Exemplaryconnections include parallel interfaces, serial interfaces, and businterfaces.

In one embodiment, the computing architecture 1100 may comprise or beimplemented as part of an electronic device. Examples of an electronicdevice may include without limitation a mobile device, a personaldigital assistant, a mobile computing device, a smart phone, a cellulartelephone, a handset, a one-way pager, a two-way pager, a messagingdevice, a computer, a personal computer (PC), a desktop computer, alaptop computer, a notebook computer, a handheld computer, a tabletcomputer, a server, a server array or server farm, a web server, anetwork server, an Internet server, a work station, a mini-computer, amain frame computer, a supercomputer, a network appliance, a webappliance, a distributed computing system, multiprocessor systems,processor-based systems, consumer electronics, programmable consumerelectronics, television, digital television, set top box, wirelessaccess point, base station, subscriber station, mobile subscribercenter, radio network controller, router, hub, gateway, bridge, switch,machine, or combination thereof. The embodiments are not limited in thiscontext.

The computing architecture 1100 includes various common computingelements, such as one or more processors, co-processors, memory units,chipsets, controllers, peripherals, interfaces, oscillators, timingdevices, video cards, audio cards, multimedia input/output (I/O)components, and so forth. The embodiments, however, are not limited toimplementation by the computing architecture 1100.

As shown in FIG. 11, the computing architecture 1100 comprises aprocessing unit 1104, a system memory 1106 and a system bus 1108. Theprocessing unit 1104 can be any of various commercially availableprocessors. Dual microprocessors and other multi processor architecturesmay also be employed as the processing unit 1104. The system bus 1108provides an interface for system components including, but not limitedto, the system memory 1106 to the processing unit 1104. The system bus1108 can be any of several types of bus structure that may furtherinterconnect to a memory bus (with or without a memory controller), aperipheral bus, and a local bus using any of a variety of commerciallyavailable bus architectures.

The computing architecture 1100 may comprise or implement variousarticles of manufacture. An article of manufacture may comprise acomputer-readable storage medium to store logic. Examples of acomputer-readable storage medium may include any tangible media capableof storing electronic data, including volatile memory or non-volatilememory, removable or non-removable memory, erasable or non-erasablememory, writeable or re-writeable memory, and so forth. Examples oflogic may include executable computer program instructions implementedusing any suitable type of code, such as source code, compiled code,interpreted code, executable code, static code, dynamic code,object-oriented code, visual code, and the like.

The system memory 1106 may include various types of computer-readablestorage media in the form of one or more higher speed memory units, suchas read-only memory (ROM), random-access memory (RAM), dynamic RAM(DRAM), Double-Data-Rate DRAM (DDRAM), synchronous DRAM (SDRAM), staticRAM (SRAM), programmable ROM (PROM), erasable programmable ROM (EPROM),electrically erasable programmable ROM (EEPROM), flash memory, polymermemory such as ferroelectric polymer memory, ovonic memory, phase changeor ferroelectric memory, silicon-oxide-nitride-oxide-silicon (SONOS)memory, magnetic or optical cards, or any other type of media suitablefor storing information. In the illustrated embodiment shown in FIG. 11,the system memory 1106 can include non-volatile memory 1110 and/orvolatile memory 1112. A basic input/output system (BIOS) can be storedin the non-volatile memory 1110.

The computer 1102 may include various types of computer-readable storagemedia in the form of one or more lower speed memory units, including aninternal hard disk drive (HDD) 1114, a magnetic floppy disk drive (FDD)1116 to read from or write to a removable magnetic disk 1118, and anoptical disk drive 1120 to read from or write to a removable opticaldisk 1122 (e.g., a CD-ROM or DVD). The HDD 1114, FDD 1116 and opticaldisk drive 1120 can be connected to the system bus 1108 by a HDDinterface 1124, an FDD interface 1126 and an optical drive interface1128, respectively. The HDD interface 1124 for external driveimplementations can include at least one or both of Universal Serial Bus(USB) and IEEE 1194 interface technologies.

The drives and associated computer-readable media provide volatileand/or nonvolatile storage of data, data structures, computer-executableinstructions, and so forth. For example, a number of program modules canbe stored in the drives and memory units 1110, 1112, including anoperating system 1130, one or more application programs 1132, otherprogram modules 1134, and program data 1136.

A user can enter commands and information into the computer 1102 throughone or more wire/wireless input devices, for example, a keyboard 1138and a pointing device, such as a mouse 1140. Other input devices mayinclude a microphone, an infra-red (IR) remote control, a joystick, agame pad, a stylus pen, touch screen, or the like. These and other inputdevices are often connected to the processing unit 1104 through an inputdevice interface 1142 that is coupled to the system bus 1108, but can beconnected by other interfaces such as a parallel port, IEEE 1194 serialport, a game port, a USB port, an IR interface, and so forth.

A monitor 1144 or other type of display device is also connected to thesystem bus 1108 via an interface, such as a video adaptor 1146. Inaddition to the monitor 1144, a computer typically includes otherperipheral output devices, such as speakers, printers, and so forth.

The computer 1102 may operate in a networked environment using logicalconnections via wire and/or wireless communications to one or moreremote computers, such as a remote computer 1148. The remote computer1148 can be a workstation, a server computer, a router, a personalcomputer, portable computer, microprocessor-based entertainmentappliance, a peer device or other common network node, and typicallyincludes many or all of the elements described relative to the computer1102, although, for purposes of brevity, only a memory/storage device1150 is illustrated. The logical connections depicted includewire/wireless connectivity to a local area network (LAN) 1152 and/orlarger networks, for example, a wide area network (WAN) 1154. Such LANand WAN networking environments are commonplace in offices andcompanies, and facilitate enterprise-wide computer networks, such asintranets, all of which may connect to a global communications network,for example, the Internet.

When used in a LAN networking environment, the computer 1102 isconnected to the LAN 1152 through a wire and/or wireless communicationnetwork interface or adaptor 1156. The adaptor 1156 can facilitate wireand/or wireless communications to the LAN 1152, which may also include awireless access point disposed thereon for communicating with thewireless functionality of the adaptor 1156.

When used in a WAN networking environment, the computer 1102 can includea modem 1158, or is connected to a communications server on the WAN1154, or has other means for establishing communications over the WAN1154, such as by way of the Internet. The modem 1158, which can beinternal or external and a wire and/or wireless device, connects to thesystem bus 1108 via the input device interface 1142. In a networkedenvironment, program modules depicted relative to the computer 1102, orportions thereof, can be stored in the remote memory/storage device1150. It will be appreciated that the network connections shown areexemplary and other means of establishing a communications link betweenthe computers can be used.

The computer 1102 is operable to communicate with wire and wirelessdevices or entities using the IEEE 802 family of standards, such aswireless devices operatively disposed in wireless communication (e.g.,IEEE 802.11 over-the-air modulation techniques) with, for example, aprinter, scanner, desktop and/or portable computer, personal digitalassistant (PDA), communications satellite, any piece of equipment orlocation associated with a wirelessly detectable tag (e.g., a kiosk,news stand, restroom), and telephone. This includes at least Wi-Fi (orWireless Fidelity), WiMax, and Bluetooth™ wireless technologies. Thus,the communication can be a predefined structure as with a conventionalnetwork or simply an ad hoc communication between at least two devices.Wi-Fi networks use radio technologies called IEEE 802.11x (a, b, g, n,etc.) to provide secure, reliable, fast wireless connectivity. A Wi-Finetwork can be used to connect computers to each other, to the Internet,and to wire networks (which use IEEE 802.3-related media and functions).

FIG. 12 illustrates a block diagram of an exemplary communicationsarchitecture 1200 suitable for implementing various embodiments aspreviously described. The communications architecture 1200 includesvarious common communications elements, such as a transmitter, receiver,transceiver, radio, network interface, baseband processor, antenna,amplifiers, filters, and so forth. The embodiments, however, are notlimited to implementation by the communications architecture 1200.

As shown in FIG. 12, the communications architecture 1200 comprisesincludes one or more clients 1202 and servers 1204. The clients 1202 mayimplement the client systems 310, 400. The servers 1204 may implementthe server system 330. The clients 1202 and the servers 1204 areoperatively connected to one or more respective client data stores 1208and server data stores 1210 that can be employed to store informationlocal to the respective clients 1202 and servers 1204, such as cookiesand/or associated contextual information.

The clients 1202 and the servers 1204 may communicate informationbetween each other using a communication framework 1206. Thecommunications framework 1206 may implement any well-knowncommunications techniques and protocols, such as those described withreference to system 1100. The communications framework 1206 may beimplemented as a packet-switched network (e.g., public networks such asthe Internet, private networks such as an enterprise intranet, and soforth), a circuit-switched network (e.g., the public switched telephonenetwork), or a combination of a packet-switched network and acircuit-switched network (with suitable gateways and translators).

FIG. 13 is a diagram of an exemplary system embodiment and inparticular, FIG. 13 is a diagram showing a platform 1300, which mayinclude various elements. For instance, FIG. 13 shows that platform(system) 1310 may include a processor/graphics core 1302 which mayinclude an applications processor, a chipset/platform control hub (PCH)1304, an input/output (I/O) device 1306, a random access memory (RAM)(such as dynamic RAM (DRAM)) 1308, and a read only memory (ROM) 1310,display electronics 1320, display backlight 1322, non-volatile memoryport 1324, antenna 1326 and various other platform components 1314(e.g., a fan, a crossflow blower, a heat sink, DTM system, coolingsystem, housing, vents, and so forth). System 1300 may also includewireless communications chip 1316 and graphics device 1318. The displayelectronics may include a liquid crystal display (LCD) screen, touchscreen display, or other display. The I/O device 1306 may include akeyboard, mouse, and/or speakers. The embodiments, however, are notlimited to these elements.

As shown in FIG. 13, I/O device 1306, RAM 1308, and ROM 1310 are coupledto processor 1302 by way of chipset 1304. Chipset 1304 may be coupled toprocessor 1302 by a bus 1312. Accordingly, bus 1312 may include multiplelines.

Processor 1302 may be a central processing unit comprising one or moreprocessor cores and may include any number of processors having anynumber of processor cores. The processor 1302 may include any type ofprocessing unit, such as, for example, CPU, multi-processing unit, areduced instruction set computer (RISC), a processor that have apipeline, a complex instruction set computer (CISC), digital signalprocessor (DSP), and so forth. In some embodiments, processor 1302 maybe multiple separate processors located on separate integrated circuitchips. In some embodiments processor 1302 may be a processor havingintegrated graphics, while in other embodiments processor 1302 may be agraphics core or cores.

It is emphasized that the Abstract of the Disclosure is provided toallow a reader to quickly ascertain the nature of the technicaldisclosure. It is submitted with the understanding that it will not beused to interpret or limit the scope or meaning of the claims. Inaddition, in the foregoing Detailed Description, it can be seen thatvarious features are grouped together in a single embodiment for thepurpose of streamlining the disclosure. This method of disclosure is notto be interpreted as reflecting an intention that the claimedembodiments require more features than are expressly recited in eachclaim. Rather, as the following claims reflect, inventive subject matterlies in less than all features of a single disclosed embodiment. Thusthe following claims are hereby incorporated into the DetailedDescription, with each claim standing on its own as a separateembodiment. In the appended claims, the terms “including” and “in which”are used as the plain-English equivalents of the respective terms“comprising” and “wherein,” respectively. Moreover, the terms “first,”“second,” “third,” and so forth, are used merely as labels, and are notintended to impose numerical requirements on their objects.

Some embodiments may be described using the expression “coupled” and“connected” along with their derivatives. These terms are not intendedas synonyms for each other. For example, some embodiments may bedescribed using the terms “connected” and/or “coupled” to indicate thattwo or more elements are in direct physical or electrical contact witheach other. The term “coupled,” however, may also mean that two or moreelements are not in direct contact with each other, but yet stillco-operate or interact with each other.

Some embodiments may be described using the expression “one embodiment”or “an embodiment” along with their derivatives. These terms mean that aparticular feature, structure, or characteristic described in connectionwith the embodiment is included in at least one embodiment. Theappearances of the phrase “in one embodiment” in various places in thespecification are not necessarily all referring to the same embodiment.Further, some embodiments may be described using the expression“coupled” and “connected” along with their derivatives. These terms arenot necessarily intended as synonyms for each other. For example, someembodiments may be described using the terms “connected” and/or“coupled” to indicate that two or more elements are in direct physicalor electrical contact with each other. The term “coupled,” however, mayalso mean that two or more elements are not in direct contact with eachother, but yet still co-operate or interact with each other.

Some embodiments may be implemented, for example, using acomputer-readable medium or article which may store an instruction or aset of instructions that, if executed by a computer, may cause thecomputer to perform a method and/or operations in accordance with theembodiments. Such a computer may include, for example, any suitableprocessing platform, computing platform, computing device, processingdevice, computing system, processing system, computer, processor, or thelike, and may be implemented using any suitable combination of hardwareand/or software. The computer-readable medium or article may include,for example, any suitable type of memory unit, memory device, memoryarticle, memory medium, storage device, storage article, storage mediumand/or storage unit, for example, memory, removable or non-removablemedia, erasable or non-erasable media, writeable or re-writeable media,digital or analog media, hard disk, floppy disk, Compact Disk Read OnlyMemory (CD-ROM), Compact Disk Recordable (CD-R), Compact DiskRewriteable (CD-RW), optical disk, magnetic media, magneto-opticalmedia, removable memory cards or disks, various types of DigitalVersatile Disk (DVD), a tape, a cassette, or the like. The instructionsmay include any suitable type of code, such as source code, compiledcode, interpreted code, executable code, static code, dynamic code,encrypted code, and the like, implemented using any suitable high-level,low-level, object-oriented, visual, compiled and/or interpretedprogramming language.

Unless specifically stated otherwise, it may be appreciated that termssuch as “processing,” “computing,” “calculating,” “determining,” or thelike, refer to the action and/or processes of a computer or computingsystem, or similar electronic computing device, that manipulates and/ortransforms data represented as physical quantities (e.g., electronic)within the computing system's registers and/or memories into other datasimilarly represented as physical quantities within the computingsystem's memories, registers or other such information storage,transmission or display devices. The embodiments are not limited in thiscontext.

What has been described above includes examples of the disclosedarchitecture. It is, of course, not possible to describe everyconceivable combination of components and/or methodologies, but one ofordinary skill in the art may recognize that many further combinationsand permutations are possible. Accordingly, the novel architecture isintended to embrace all such alterations, modifications and variationsthat fall within the spirit and scope of the appended claims.

Although the subject matter has been described in language specific tostructural features and/or methodological acts, it is to be understoodthat the subject matter defined in the appended claims is notnecessarily limited to the specific features or acts described above.Rather, the specific features and acts described above are disclosed asexample forms of implementing the claims.

1. An apparatus, comprising: a transmitter arranged to wirelesslytransmit channel status reports for channels within a transmission bandto a base station; a processor; a rank adaptation (RA) module operableon the processor to direct the transmitter to send: a multiplicity ofsub-band channel quality indicator (CQI) reports, each sub-band CQIreport comprising a measurement of a respective sub-band of thetransmission band; and a multiplicity of rank indicator (RI) reports,each sub-band CQI report to be accompanied by an RI report; and adigital display arranged to display information transmitted via the basestation to the apparatus.
 2. The apparatus of claim 1, the RA modulearranged to direct the transmitter to transmit a multiplicity ofsub-band precoding matrix index (PMI) reports, each sub-band PMI reportcorresponding to a respective sub-band CQI report.
 3. The apparatus ofclaim 1, the RA module arranged to direct the transmitter to transmiteach sub-band CQI report periodically over a CQI report period.
 4. Theapparatus of claim 3, each CQI report period comprising a CQI reportfrom each scanned sub-band of the transmission band, each CQI reporttransmitted at a different instance from each other CQI report.
 5. Theapparatus of claim 3, the RA module arranged to direct the transmitterto transmit a sub-band RI report with a respective sub-band CQI report,each sub-band RI report and respective sub-band CQI report correspondingto a same sub-band of the transmission band.
 6. The apparatus of claim1, the RA module arranged to direct the transmitter to send the sub-bandCQI reports aperiodically.
 7. The apparatus of claim 6, the RA modulearranged to direct the transmitter to send the RI reports as amultiplicity of band part RI reports, each band part RI reportcomprising a rank indicator determined from multiple contiguoussub-bands of the transmission band.
 8. The apparatus of claim 6, the RAmodule arranged to direct the transmitter to send the RI reports as amultiplicity of sub-band RI reports, each sub-band RI report based upona rank indicator determined from a single sub-band of the transmissionband.
 9. An apparatus, comprising: a receiver arranged to receivewireless downlink communications comprising one or more data streamsover a transmission band, each communication characterized by a rank rspecifying a number of data streams to be simultaneously communicatedover the transmission band; a memory containing a multiple rankcodebook; a processor; and a rank adaptation module (RA) operable on theprocessor to: determine a precoding matrix index i₀ for lowest ranktransmissions; set a precoding matrix index i_(r) for a higher levelrank transmission based upon i₀; and perform rank adaptation to select arank for receiving the downlink communications.
 10. The apparatus ofclaim 9, the processor arranged to determine i₀ according toi₀ =  _(v_(i) ∈ C₀)^(argmax)(trace(v_(i)^(H)Rv_(i))) where C₀ is acodebook having rank
 1. 11. The apparatus of claim 9, the processorarranged to select a rank r_(best) according tor_(best) =  _(0 < r < r_(max))^(argmax)(capacity(v_(i_(r)), H, SINR)).12. The apparatus of claim 9, the processor arranged to perform the rankadaptation after receiving a signal from a base station indicating apreference of MIMO mode operation.
 13. An article, comprising acomputer-readable storage medium containing instructions that ifexecuted by a processor enable a system to: determine a precoding matrixindex for lowest rank transmissions; set a precoding matrix index for ahigher level rank transmission; and select a rank based upon thedetermined precoding matrix index for lowest rank transmissions.
 14. Thearticle of claim 13 comprising instructions that when executed by aprocessor enable the system to determine a lowest rank PMI i₀ accordingto i₀ =  _(v_(i) ∈ C₀)^(argmax)(trace(v_(i)^(H)Rv_(i))) where C₀ is acodebook having rank
 1. 15. The article of claim 13 comprisinginstructions that when executed by a processor enable the system toselect a rank r_(best) according tor_(best) =  _(0 < r < r_(max))^(argmax)(capacity(v_(i_(r)), H, SINR)).16. The article of claim 13 comprising instructions that when executedby a processor enable the system to perform a rank adaptation inresponse to a received control signal from a base station indicating apreference of multiple input-multiple output (MIMO) mode.
 17. Anapparatus, comprising: a receiver arranged to receive wireless downlinkcommunications comprising one or more data streams over a transmissionband, each communication characterized by a rank r specifying a numberof data streams to be substantially simultaneously communicated over thetransmission band; a memory containing a multiple rank codebook; aprocessor; and a rank adaptation (RA) module operable on the processorto: search the multiple rank codebook and determine a precoding matrixindex (PMI) i_(r) for each of a multiplicity of ranks; select a rankr_(best) to be used in the downlink communications; and direct atransmitter to wirelessly report the selected rank r_(best) and acorresponding value i_(r) for use in the wireless downlinkcommunications.
 18. The apparatus of claim 17, the processor arranged todetermine i_(r) according toi_(r) =  _(v_(i) ∈ C_(r))^(argmax)(trace(v_(i)^(H)Rv_(i))).
 19. Theapparatus of claim 17, the processor arranged to determine r_(best)according tor_(best) =  _(0 < r < r_(max))^(argmax)(capacity(v_(i_(r)), H, SINR)).20. The apparatus of claim 17, the processor arranged to perform a rankadaptation in response to a received control signal from a base stationindicating a preference of MIMO mode.
 21. An article, comprising acomputer-readable storage medium containing instructions that ifexecuted by a processor enable a system to: search a multiple rankcodebook and determine a precoding matrix index (PMI) i_(r) for each ofa multiplicity of ranks, each rank specifying a number of data streamsto be simultaneously communicated over a transmission band in a downlinkcommunication; select a rank r_(best) to be used in the downlinkcommunication; and direct a transmitter to wirelessly report theselected rank r_(best) and a corresponding value i_(r) for use in thewireless downlink communications.
 22. The article of claim 21 comprisinginstructions that when executed by a processor enable the system todetermine i_(r) according toi_(r) =  _(v_(i) ∈ C_(r))^(argmax)(trace(v_(i)^(H)Rv_(i))).
 23. Thearticle of claim 21 comprising instructions that when executed by aprocessor enable the system to determine r_(best) according tor_(best) =  _(0 < r < r_(max))^(argmax)(capacity(v_(i_(r)), H, SINR)).24. The article of claim 21 comprising instructions that when executedby a processor enable the system to perform a rank adaptation inresponse to a received control signal from a base station indicating apreference of MIMO mode.